The present invention relates generally to power sharing among multiple power sources and in particular to power sharing among power sources providing power to a material handling system using a droop sharing method.
Automated materials transport systems (AHMS), are known for moving materials among various work stations of a facility, typically under the control of a central computer. Such systems are employed, for example, in semiconductor fabrication facilities for moving semiconductor wafers to successive work stations. In one type of wafer transport system, a monorail track is routed past the various work stations and a plurality of electrically powered material transport vehicles (MTVs) are mounted on the track and are moveable thereon. The MTV delivers the wafers to a work station for processing and removes the wafers from the work station after the requisite processing operations have been completed. In general, the monorail track is composed of a series of interconnected track sections. This sections may include one or more routing sections that are operative to provide plural paths along the track. In addition, the track may serve as a conduit for the conductors that supply the power to the AMHS.
As discussed above, MTVs are often used in manufacturing and warehouse environments for transporting and manipulating articles of manufacture. Such vehicles are desirable in these environments due to their clean operation and low noise. The MTVs are propelled along the monorail track by an electric motor and are under the control of a central control system. The electric motor and other electrical equipment onboard the MTV may be powered by an onboard energy source such as a battery, ultracapacitor, fuel cell, or fly-wheel. These onboard energy sources receive power periodically from the AMHS power sources either by a direct electrical connection or by an inductive power transfer system. Alternatively, the motor and other electrical equipment onboard the MTV may be powered by an external power source continuously coupled to the MTV. The MTV can receive the electrical power from the AMHS power source via a direct electrical contact system or alternatively, from an inductive power transfer system.
The AMHS power source must be capable of providing sufficient current to power the entire AHMS system under all possible conditions. The AMES system typically includes a one or more power sources, a fleet of electrically powered MTVs and various electrically powered equipment at each of the nodes. In general, a node in an AMHS is a location where a MTV is stopped, loaded, unloaded, or redirected. As such, a node may be a workstation or an intersection of one or more tracks where the vehicle may be redirected. At a work station, power is needed to automatically load or unload the MTV, whether the MTV provides the mechanism or the work station provides the mechanism for the loading or unloading process. A node at the intersection of two or more tracks within the AMHS requires power to redirect the MTV either to a new level of the monorail track using an elevator type mechanism or onto a new track, on the same level, using a turntable type mechanism. Thus, every MTV and every node contribute to the power requirements of the AMHS.
The physical size of the AMHS also contributes to the large power consumption of the system. Often the layout of an AMHS will be in the shape generally of an oval. The oval may also have side loops that intersect the main oval at two or more intersections. The major axis of the main oval may be several hundred feet long to over a thousand feet long. Each of the side loops may be on the order of a few hundred feet long. Work stations are located throughout the AMHS, often with multiple workstations being located on an individual side loop. The layout of the AMHS typically therefore includes nodes and workstations that are physically remote from one another and from a power source. In order to provide power to these nodes, workstations, and MTVs that are physically remote, the AMHS contains long conductor runs. Long conductor runs can result in substantial power lost in the conductors due to the electrical resistance inherent in the conductors. Moreover, voltage provided by a power source will decrease over a long conductor run also due to the electrical resistance inherent in the conductor. This power loss and voltage drop can cause a variety of equipment problems that could negatively impact the efficient working of the AMHSn such as damage to the MTV onboard motors.
A single AMHS power source would therefore need to have a sufficiently large current supply capacity to ensure the ability to provide sufficient current to the entire AMHS system under worst case scenarios. These scenarios could include times when a majority the MTVs are located at the AMHS nodes and workstation locations that are the most physically remote from the single AMHS power source. Thus, the single AMHS power source would have to provide sufficient current not only to power the fleet of MTVs and the equipment contained at each node, but must be able to provide additional current to overcome the resistance losses in the conductors as well. However the AMHS will draw this large current only rarely. Therefore, under most circumstances, the increased size, increased cost, increased amount of heat generated, and the increased complexity of a single large AMHS power source would not be needed. Furthermore, the loss of the single AMHS would constitute a one-point failure mechanism that would disable the entire AMHS.
A prior art solution has been to divide the AMHS into a plurality of individual power zones. Each power zone has a corresponding individual power source that provides power to that zone. Each individual power source must be able to provide sufficient current not only for each of the nodes and the equipment associated with the node, but must also be able to supply the current necessary to power the maximum number of MTVs that may be operating within the node as well. Therefore, larger more costly power sources will still be needed to ensure system operation under this condition, As with the single power supply this leads to an increase in the size, cost, and the amount of heat generated within the system that must be dissipated. Also, the failure of a single power source will disable the entire AMHS. Thus even with the division of the AMHS into individual power zones, a failure of a single power source is a one-point failure mechanism that will disable the entire AMHS.
It would be desirable therefore to provide power to an AMHS using a plurality of remote power sources in a manner in which the failure of a single power source will not disable the entire AMHS and in which the cost, size, and complexity of the power sources is reduced.